Considerations involving several factors regarding the present invention are discussed in separate labeled sections below. In particular, in connection with the fluid pressure reduction device of the present invention, the relevant considerations discussed separately below involve (A) Aerodynamic Noise; (B) Manufacturing; and (C) Hydrodynamic Noise.
(A) Aerodynamic Noise
In the control of fluid in industrial processes, such as oil and gas pipeline systems, chemical processes, etc., it is often necessary to reduce the pressure of a fluid. Adjustable flow restriction devices such as flow control valves and fluid regulators and other fixed fluid restriction devices such as diffusers, silencers, and other back pressure devices are utilized for this task. The purpose of the fluid control valve and/or other fluid restricting device in a given application may be to control flow rate or other process variables, but the restriction induces a pressure reduction inherently as a by-product of its flow control function.
Pressurized fluids contain stored mechanical potential energy. Reducing the pressure releases this energy. The energy manifests itself as the kinetic energy of the fluid--both the bulk motion of the fluid and its random turbulent motion. Turbulence is the chaotic motion of a fluid. However there is momentary structure in this random motion. Turbulent eddies (vortices) are formed, but rapidly break down into smaller eddies which in turn also breakdown, etc. Eventually viscosity damps out the motion of the smallest eddies and the energy has been transformed into heat.
This turbulent fluid motion has associated pressure and velocity fluctuations that act upon the structural elements of the piping system causing vibration. Vibration is undesirable because it can (if sufficiently strong) lead to fatigue failure of pressure retaining components or other types of wear, degradation of performance, or failure of attached instruments, etc. Even when not physically damaging, vibration generates air-borne noise that is annoying to or may damage the hearing of people.
There are three basic methods for noise control:
1.) Limit the amount of vibration generated initially. Since the amount of energy being dissipated is set by the application, this reduction in noise level must come from reducing the efficiency of fluid energy to acoustic energy conversion. PA1 2.) Absorb the acoustic energy. A typical example of an industrial device is a fiber-glass packed silencer. PA1 3.) Block the transmission of the sound. An example would be a thick-walled pipe. PA1 1.) Reduce the pressure in small steps or stages rather than by a single, turbulence generating process. Typically a pressure reducing stage is accomplished by a flowstream contraction/expansion pair or by a direction change. In either case a higher velocity fluid jet is formed and is surrounded by a lower velocity region. The resultant turbulent mixing generates noise. If the pressure change across the stage is sufficiently high, the jet will "choke" or reach sonic velocity and shocks will form in the flow stream. A shock produces a sudden change in the flow's thermodynamic state. For example, the pressure may drop appreciably. When incoming turbulence passes through a shock, broadband shock-associated noise is also generated. PA1 2.) Avoid the contact of high speed jets and turbulence onto solid surfaces. The so-called Reynolds stresses in free stream turbulence are a source of noise. However, when turbulence contacts a solid surface, acoustic dipole sources result. Dipole sources are fairly effective noise sources when the mean stream velocity is low. PA1 3.) Subdivide the flow stream into small streams. This strategy actually accomplishes multiple desirable results. Due to their smaller characteristic dimensions, small streams create higher frequency turbulence because the initial eddies are smaller. The energy has been moved ahead in the eddy breakdown process, thus skipping opportunities for vibration generation. Secondly, these smaller eddies contain most of their energy in a frequency range that is less readily absorbed (and then radiated as noise) by piping components. Thus small streams improve the effectiveness of typical industrial piping to block the transmission of the noise that is generated. Thirdly, the human ear is less sensitive to high frequency noise, so an apparent noise reduction occurs. Fourth, it is easier to keep these small jets from impinging on a solid surface thus minimizing the dipole type noise. Finally, as long as jets from each stream remain isolated, the noise from each remains uncorrelated with the others and the total is minimized--similar to the effect of staging. Plugging of passages by fluid-borne debris establishes a practical lower limit for stream size. PA1 4.) Combinations of the above strategies. One problem with staging for compressible flow is that as pressure is reduced, the volume of flow in subsequent stages is increased. For high pressure ratio (inlet pressure/outlet pressure) applications the increase in required flow area can be substantial. Many prior fluid restriction devices utilize passages with increasing flow area. For compressible flows these restrictors are normally used so that flow is radially outward through the annular cage wall. This takes advantage of the natural increase in gross cross-sectional area to provide space for increased passage area. PA1 1) Minimizes aerodynamic noise generation by constructing flow passage geometry that advantageously controls flow separation, shock formation, pressure recovery, and fluid turbulence characteristics. PA1 2) For liquid flows, minimizes the propensity for cavitation by constructing flow passage geometry that controls flow separation and pressure recovery. PA1 3) Implements this desirable flow passage geometry from a standard raw material form to reduce inventory and shorten delivery. PA1 4) Implements this desirable flow passage geometry in a device that can be cost effectively manufactured by modern techniques--CNC controlled laser or water jet cutting, etc. PA1 5) Minimizes overall valve cost by shrinking the size of the pressure reducing element as compared to tortuous path principle designs currently utilized. PA1 6) Provides a fluid control valve with a smoothly varying resistance element with respect to plug position so as to improve control performance. PA1 7) Provides a cost effective means to rigidly assemble a stack of disks during manufacture and use that also allows disassembly for repair or cleaning. PA1 8) Provides a fluid control element which can be tailored to special applications without expensive tooling costs.
The portion of the total amount of power that is converted into vibration depends on the nature of the flow field and the turbulence, in addition to the response or willingness of the surrounding structure to absorb that energy. The fraction of mechanical power converted to noise is known as the acoustical conversion efficiency.
There are several known methods to minimize the noise and vibration generated by reducing fluid pressure. In gases the four often-used methods are:
The main technical challenge of reducing the noise and vibration generated by reducing fluid pressure is the cost effective implementation of flow path geometry that manipulates the fluid state most effectively.
(B) Manufacturing
Forming the desired passageways in low-noise restricting elements is typically very expensive. The proper raw material form also affects cost and delivery. Annular castings or bar can be used to make the cylindrical structures in much of the currently available flow restrictors--sleeves, rings, etc. However, this requires many combinations of diameter, length, and thickness for the raw material. Annular disks of many ID/OD combinations can be cut from a common sheet and stacked to the desirable height. Wrought forms like sheet are less likely to contain defects, such as porosity, than are annular castings.
Historically, disks used in stacks to form a cage have been manufactured by chemical etching, milling, electron-discharge machining (EDM), casting, cutting, punching, or drilling. Chemical etching is a versatile process but is very expensive for parts of the size needed for valve cages. Furthermore, the acid bath and the dissolved metals in it presents a hazardous waste disposal problem. Milling is expensive and has limitations for small features due to a practical lower limit on the cutter size. Wire EDM is limited to through-cut designs and is slow. Plunge EDM can make recess-type designs but is better suited for pattern making, rather than mass-producing the disks. Casting is inexpensive but requires an expensive hardware pattern for each version of the design. Castings may require flattening and/or grinding operations prior to assembling the stack. Punching is limited to through-cut designs, requires a unique die for each punched shape, and the disks may not be flat after the stamping operation. Die wear can degrade the flow manipulating characteristics of the desired passage shape. Furthermore, small features may not be possible, especially for thick disks. Drilling limits passage shape to axisymmetric holes and tapers. Additionally, radii cannot be put on the inside of an annular cage structure by drilling.
Cutting methods include plasma, laser, and erosive water jet. Clearly these methods are limited to through-cut designs. However, many of the through-cut designs in prior flow restrictors do not lend themselves to cost effective production by cutting. For example, the skeletal disks shown in Self (U.S. Pat. No. 3,513,864) require a huge number of starts and stops of flame/beam/jet as the operation moves from one cutout region to the next. This starting and stopping adds substantial machine time per part, driving part cost proportionally higher. It is desired to provide a disk design that could be efficiently made with a cutting process.
Additionally, the widespread availability of computer numerical controlled (CNC) machines, computer aided design (CAD) systems, and automated interfaces in-between has dramatically shifted the relative cost advantage of software (CNC cutting) versus physical pattern-based manufacturing processes (casting). This software-based tooling is especially advantageous for severe service applications requiring noise control type restrictions that are often specially designed for the particular application.
Typically, disk stacks are held together by brazing or bolting. Sometimes disk to disk joints are individually welded.
In addition to the restrictor element cost, the size of the element for a given flow capacity influences the size of the valve body required which in turn greatly influences overall valve cost.
Traditional tortuous path trims have purposefully inefficient flow passageways to distribute the pressure reduction. Hence the flow rate per unit cross sectional area is less than for example a two-stage device. Consequently a traditional tortuous path based restrictor must be significantly larger to accommodate both the additional passage area and the extra stages per passage. This increase in restrictor size translates into a very large, heavy, expensive valve body requiring a large actuator to operate the valve.
(C) Hydrodynamic Noise
While the physical phenomena responsible for the generation of hydrodynamic noise in liquid pressure reduction is different, many of the fabrication techniques of this invention are also advantageous for use in liquid passageways. In industrial applications the chief source of noise and vibration from the pressure reduction of liquids is cavitation. Cavitation is caused in a flow stream when the fluid passes through a zone where the pressure is below its vapor pressure. Vapor bubbles form and then collapse after traveling downstream into a zone where pressure exceeds the vapor pressure. The collapse process may cause noise, vibration, and material attack.
One method to avoid these problems is to design a passageway in which the pressure never dips below vapor pressure. As with gas flows, multiple stages are often used. The number required depends on the amount of pressure reduction allocated to each stage and the minimum pressure in each stage as compared to its overall pressure change, i.e. the amount of pressure recovery. Low pressure recovery is desirable. Right angle turn-based stages often found in stacked plate flow restrictors exhibit pressure recovery. Consequently more turns are required; increasing the complexity, size and cost of the valve assembly.
As a matter of practicality, it is advantageous to take the largest pressure drop in the first stage (where static pressure is the highest) and progressively smaller pressure drops on subsequent stages. This approach is sometimes described as an increasing area flow path when applied to direction-change based stages.
As with compressible flows, small passage size is beneficial. Often it is permissible to operate under conditions that produce small amounts of cavitation. A group of small isolated two-phase jets is less efficient at exciting vibration than is a large two-phase jet.
As a theoretical principle, the control of velocity is an indirect means to control vibration and noise in liquids. The purpose of velocity control is to minimize the Bernoulli effect that reduces the local static pressure of a fluid due to its overall bulk motion. This relatively higher static pressure in turn minimizes the range of pressure conditions that cause cavitation.
It is therefore desired to provide a fluid pressure reduction device having low acoustical conversion efficiency or hydrodynamic noise, and which can be most efficiently manufactured to lower manufacturing costs.